1. Field of the Invention
The present invention relates to a master medium which bears information, and is used in a process of magnetically transferring the information from the master medium to a slave medium.
2. Description of the Related Art
Recently, the amount of information which is required to be handled and stored is increasing. Therefore, there are demands for an inexpensive magnetic recording medium which can store a large amount of information, and be preferably accessed at high speed so that an arbitrary portion of information can be read out quickly. The high-density flexible disk is known as an example of such a magnetic recording medium.
The so-called tracking servo technique plays an important role in realizing a high-density flexible disk having great capacity. Due to the tracking servo technique, a magnetic head can move precisely within a narrow track width, and a signal can be regenerated with a high S/N ratio. A so-called preformat including tracking servo signals, address information signals, regeneration clock signals, and the like is recorded on the track of the disk in such a manner that the signals constituting the preformat appear at predetermined intervals during each rotation of the disk. The magnetic head is arranged so that the magnetic head can move precisely on the track by reading the signals in the preformat and correcting its own position.
Conventionally, the preformat is produced by recording the signals on each disk on a track-by-track basis by using a dedicated servo recording apparatus. However, it takes a long time to record the preformat by using the servo recording apparatus. Therefore, the cost of the preformat recording forms a considerable proportion of the total manufacturing cost, and thus reduction of the preformat recording cost is required.
On the other hand, Japanese Unexamined Patent Publications Nos. 63 (1988)-183623, 10 (1998)-40544, and 10 (1998)-269566 disclose techniques of recording a preformat by magnetic transfer, instead of recording signals constituting the preformat on the track-by-track basis. However, the above patent publications do not substantially disclose concrete procedures. In particular, conditions of the magnetic fields which are applied during magnetic transfer and constructions of apparatuses for generating the magnetic fields are not disclosed.
For example, JUPP63 (1988)-183623 and JUPP10 (1998)-40544 disclose methods of recording magnetization patterns corresponding to information signals on a magnetic recording medium (as a slave medium) In the disclosed methods, a master medium is prepared for magnetic transfer. The master medium is produced by forming relieved portions having shapes corresponding to the information signals on a surface of a substrate, and further forming a thin magnetic film at least on the relieved portions of the substrate. The slave medium has the form of a sheet or disk, and includes a thin ferromagnetic film or a magnetic-powder layer. The magnetic transfer is achieved by placing the slave medium in close contact with the magnetic film of the master medium, and applying an AC or DC bias magnetic field to the master medium so as to excite the magnetic material (thin film) formed on the relieved portions of the master medium. Thus, the magnetization patterns corresponding to the relieved portions of the master medium are recorded on the slave medium.
Since, according to the above methods of recording magnetization patterns by magnetic transfer, the relieved portions of the master medium are placed in close contact with the slave medium which is to be preformatted, and the magnetic material on the relieved portions are concurrently excited so as to record the predetermined patterns on the slave medium, the patterns can be statically recorded on the slave medium without changing the relative positions of the master medium and the slave medium. Thus, the preformat can be precisely recorded, and the time needed for the preformatting is very short.
However, when the above methods of recording magnetization patterns by magnetic transfer are used for recording servo signals on a magnetic recording medium (as a slave medium), it is necessary to precisely form servo patterns at respectively appropriate positions on the entire area of a magnetic recording medium, where the servo patterns each have a size of the order of one micrometer or less. For example, the 3.5-inch and 2.5-inch magnetic recording mediums have diameters of 3.5 and 2.5 inches, respectively. Since the servo patterns respectively indicate addresses of information items, the respective servo patterns must be different.
Micropatterns as mentioned above can be recorded by the lithography techniques which are conventionally used in manufacturing semiconductor devices or magnetic heads. However, in the lithography techniques, an original image is reduced in order to increase accuracy. Therefore, the area which can be exposed by each exposure shot is limited to about 2 cm square. When a large-size pattern is recorded by repeating the exposure of the 2 cm square, the large-size pattern includes an array of identical patterns. That is, it is difficult to form the servo patterns on a magnetic recording medium by using the above lithography techniques since the servo patterns are respectively different.
In addition, in the methods of recording magnetization patterns by magnetic transfer, the master medium is placed in close contact with the slave medium during the magnetic transfer operation. Therefore, when the magnetic transfer operation is performed a number of times, the relieved patterns representing information wear down, and therefore the accuracy of the magnetic transfer decreases. Further, when dust exists between the master medium and the slave medium, the dust may scratch the surfaces of the relieved portions of the master medium. In these cases, the master medium must be replaced. Therefore, it is desirable that the master medium is easy to produce, and inexpensive. However, the production of the master medium by repetition of the micropattern exposure requires complicated quality control operations, and is disadvantageous in qualitative stability and production cost.
An object of the present invention is to provide a master medium for use in magnetic transfer, which is inexpensive and easy to produce.
(1) According to the first aspect of the present invention, there is provided a master medium for use in magnetic transfer of information. The master medium comprises a metal disk which has a first relief or recess pattern representing the information. The metal disk is produced by exposing a photoresist film formed on a base disk with a laser or electron beam modulated with the information while rotating the base disk, developing the photoresist film so as to form an original disk having a second relief or recess pattern, depositing metal on the original disk so as to mold the metal disk on the original disk, and removing the metal disk from the original disk.
The master medium according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (x).
(i) The original disk may be formed by etching the base disk after the photoresist film is developed.
(ii) The base disk may be made of glass or quartz.
(iii) The original disk may be made of a material containing Ni as a main component.
(iv) Preferably, the first relief or recess pattern has a height or depth of 80 to 800 nm. More preferably, the height or depth of the first relief or recess pattern is 150 to 600 nm.
(v) The master medium may further comprise a soft magnetic layer formed on the first relief or recess pattern.
(vi) The master medium having the feature (v) may further comprise a nonmagnetic layer formed between the soft magnetic layer and the first relief or recess pattern.
(vii) In the master medium having the feature (v), preferably, the soft magnetic layer has a thickness of 50 to 500 nm. More preferably, the thickness of the soft magnetic layer is 150 to 400 nm.
(viii) The master medium may further comprise a diamond-like carbon protection layer as an uppermost layer.
(ix) Preferably, the first relief or recess pattern is elongated in a radial direction. More preferably, the size of the first relief or recess pattern is in the range of 0.3 to 20 micrometers in the radial direction, and in the range of 0.2 to 5 micrometers in the circumferential direction In particular, when the first relief or recess pattern represents a servo signal, it is preferable that the first relief or recess pattern is elongated in a radial direction, and the size of the first relief or recess pattern is within the above ranges.
(x) The deposition of metal may be performed in accordance with one of various film formation methods including electroless plating, electroforming, sputtering, and ion plating.
(2) According to the second aspect of the present invention, there is provided a master medium for use in magnetic transfer of information. The master medium comprises a metal disk which has a first relief or recess pattern representing the information. The metal disk is produced by exposing a photoresist film formed on a base disk with a laser or electron beam modulated with the information while rotating the base disk, developing the photoresist film so as to form a first original disk having a second relief or recess pattern, depositing metal on the first original disk so as to mold on the first original disk a second original disk having a third relief or recess pattern, removing the second original disk from the first original disk, depositing metal on the second original disk so as to mold the metal disk on the second original disk; and, removing the metal disk from the second original disk.
The master medium according to the second aspect of the present invention may also have one or any possible combination of the aforementioned additional features (ii) to (x) and the following additional features (xi) and (xii).
(xi) The first original disk may be formed by etching the base disk after the photoresist film is developed.
(xii) The second original disk may be formed by pressing a resin solution on the first original disk, curing the resin solution so as to mold the second original disk on the first original disk, instead of depositing metal on the first original disk.
(3) According to the third aspect of the present invention, there is provided a master medium for use in magnetic transfer of information. The master medium comprises a metal disk which has a first relief or recess pattern representing the information. The metal disk is produced by exposing a photoresist film formed on a base disk with a laser or electron beam modulated with the information while rotating the base disk, developing the photoresist film so as to form a first original disk having a second relief or recess pattern, depositing metal on the first original disk so as to mold on the first original disk a second original disk having a third relief or recess pattern, removing the second original disk from the first original disk, depositing metal on the second original disk so as to mold on the second original disk a third original disk having a fourth relief or recess pattern, removing the third original disk from the second original disk, depositing metal on the third original disk so as to mold the metal disk on the third original disk, and removing the metal disk from the third original disk.
The master medium according to the third aspect of the present invention may also have one or any possible combination of the aforementioned additional features (ii) to (xii) and the following additional feature (xiii).
(xiii) The third original disk may be formed by pressing a resin solution on the second original disk, curing the resin solution so as to mold the third original disk on the second original disk, instead of depositing metal on the second original disk.
(4) The magnetic transfer is achieved by placing a slave medium in contact with the first relief or recess pattern of each master medium, and applying a magnetic field to the master medium for transferring the information represented by the first relief or recess pattern to the slave medium. For example, it is preferable that the magnetic transfer is made through the following process by using a magnetic recording medium (as a slave medium) and a master medium in which the metal disk is made of a ferromagnetic material, or the first relief or recess pattern is coated with the soft magnetic layer. The process comprises the steps of: (a) performing an initial direct-current magnetization of the magnetic recording medium in a first direction (e.g., along a track of the magnetic recording medium); (b) placing the master medium and the magnetic recording medium so that the magnetic recording medium is in contact with the first relief or recess pattern made of the ferromagnetic material or the soft magnetic layer formed on the first relief or recess pattern of the master medium; and (c) applying to the master medium a magnetic field in a second direction approximately opposite to the first direction.
Regardless of whether the first relief or recess pattern of the metal disk is a true copy or a reverse copy of the second relief or recess pattern formed on the aforementioned photoresist film, i.e., regardless of whether the pattern formed on the metal disk is a relief (positive) pattern or a recess (negative) pattern, the pattern formed on the magnetic recording medium is magnetized in the same way when the directions of the initial magnetization of the magnetic recording medium and the magnetic field applied to the master medium for magnetic transfer are inverted accordingly.
The master mediums according to the first to third aspects of the present invention are made with a metal disk which has a relief or recess pattern representing information signals to be magnetically transferred. Thus, the master mediums used in magnetic transfer of the information signals (such as servo signals) to a magnetic recording medium can be produced at low cost with a desired accuracy. In particular, a great number of identical metal disks can be produced by metal deposition using one original plate. Therefore, the qualitative stability of the magnetic transfer can be maintained by timely replacing the master medium in response to increase in the number of the magnetic transfer operations.
In addition, the master mediums according to the first to third aspects of the present invention is advantageous in their hardness, formability, and weather resistance when the metal disk in the master medium is made of a material containing Ni as a main component.
Further, when the metal disk is made of a material containing Ni as a main component, the magnetic transfer is possible without additional provision, since Ni is a ferromagnetic material. However, it is preferable to provide a soft magnetic layer even when the metal disk is made of a material containing Ni, since the transfer characteristics are improved by the provision of the soft magnetic layer. It is more preferable that a nonmagnetic layer is provided between the metal disk and the soft magnetic layer in order to eliminate the influence of the magnetism of the metal disk.
When the metal disk is made of a nonmagnetic material, the soft magnetic layer which realizes desirable transfer characteristics is indispensable.
Further, when a diamond-like carbon protection layer is provided as an uppermost layer of the master medium, the contact durability is increased, so that a great number of magnetic transfer operations can be performed by using one master medium.